1. Field of the Invention
The present invention relates to a radiation imaging apparatus for taking a radiographic image from radiation passed through an object, a control method of the radiation imaging apparatus, and a radiation imaging system having the radiation imaging apparatus.
2. Description Related to the Prior Art
In a medical field, a radiation imaging system, for example, an X-ray imaging system using X-rays is widely known. The X-ray imaging system is constituted of an X-ray generating apparatus for applying the X-rays to an object (a body portion, for example, a chest of a patient), and an X-ray imaging apparatus for taking an X-ray image by reception of the X-rays passed through the object.
In recent years, the X-ray imaging apparatus that uses a flat panel detector (FPD) as a detection panel, instead of an X-ray film or an imaging plate (IP), becomes widespread. The FPD has a matrix of pixels each of which produces and accumulates signal charge in accordance with an X-ray dose applied thereto. The FPD converts the signal charge of the pixels into a voltage signal by its signal processing circuit. Thereby, the FPD electrically detects the X-ray image, and outputs the X-ray image as digital image data.
With the aim of reducing X-ray exposure of a patient and improving X-ray image quality, some X-ray imaging systems have an automatic exposure control (AEC) function for automatic control of an X-ray dose. For example, Japanese Patent Laid-Open Publication No. 09-055298 discloses an X-ray fluoroscopic apparatus having a video camera, being an image detector, and an image intensifier disposed in front of the video camera. In this apparatus, the image intensifier converts an X-ray image into an optical image, and the video camera captures a moving image to be displayed on a monitor. The image intensifier is in the shape of a rectangle, and is provided with three intensity detection sensors that are disposed at the middle of an upper portion of the rectangle and at the right and left of a lower portion of the rectangle, respectively, as X-ray sensors for detecting the X-ray dose. The video camera captures the moving image of an object in observation, so a fluoroscopic image is displayed on the monitor. During the display (fluoroscopy), pixel values of the fluoroscopic image are detected and a histogram of the pixel values is produced in each individual area corresponding to the position of each X-ray sensor. Based on the histograms, an unexposed area to which no X-ray is applied, an object area, and a directly exposed area to which the X-rays are directly applied without through the object are determined. Out of the three X-ray sensors, the one disposed in the object area is chosen. In taking an X-ray image by use of an X-ray film, X-ray exposure time is controlled based on the X-ray dose detected by the chosen X-ray sensor.
Also, U.S. Pat. No. 7,433,445 discloses a radiation imaging apparatus for mammography. This apparatus includes an image detector composed of an FPD, and an exposure control sensor disposed in a position corresponding to an outer edge of the image detector. In this apparatus, an X-ray dose (necessary dose) necessary for obtaining desirable image equality is calculated from the thickness of an object, X-ray absorptance, and the like, prior to performing mammography. During the mammography, an X-ray dose (detected dose) detected by the exposure control sensor is compared with the necessary dose. When the detected dose has reached the necessary dose, X-ray emission is stopped.
The X-ray fluoroscopic apparatus of the Japanese Patent Laid-Open Publication No. 09-055298 is provided with the plurality of X-ray sensors for use in AEC. The X-ray sensors are used for identification of the object area, out of the three areas of the unexposed area, the object area, and the directly exposed area. The X-ray sensor has low spatial resolution, and its detection surface has fixed size and is in a fixed position. Depending on the type, size, shape, or the like of a body portion, there may be cases where AEC cannot be performed appropriately, and desirable image quality cannot be obtained.
This is because, for example, in chest radiography, the object area includes lung fields, a mediastinum, and a diaphragm that have different X-ray transmittances from each other. The difference in the X-ray transmittance causes variations in the X-ray dose to be transmitted. Therefore, the image quality differs depending on which part is used for detecting the X-ray dose as a reference of AEC. In general, the higher the density, the finer the graininess of an X-ray image would be. Thus, a part having a low X-ray transmittance is more preferably used for detecting the X-ray dose than a part having a high X-ray transmittance, because increase in the density of the entire image facilitates improving the image quality.
However, in the Japanese Patent Laid-Open Publication No. 09-055298, the plurality of X-ray sensors are disposed in the fixed positions, and have the fixed size and the low spatial resolution. Thus, the X-ray sensors may not be able to detect the X-ray dose at an appropriate position in the object area, depending on the type, size, shape, or the like of the body portion. Therefore, the apparatus of the Japanese Patent Laid-Open Publication No. 09-055298 may fail to perform AEC appropriately, depending on the type, size, shape, or the like of the body portion.
On the other hand, in the U.S. Pat. No. 7,433,445, there is only one exposure control sensor provided at the outer edge of the image detector, and its detection surface has fixed size and position. Thus, as with the Japanese Patent Laid-Open Publication No. 09-055298, the apparatus of the U.S. Pat. No. 7,433,445 may fail to perform appropriate AEC depending on the type, size, shape, or the like of the body portion, and fail to obtain favorable image quality.
Furthermore, both the X-ray sensors of the Japanese Patent Laid-Open Publication No. 09-055298 and the exposure control sensor of the U.S. Pat. No. 7,433,445 are provided separately from the image detector (video camera or FPD), and hence may cause complex structure.